An electrolyte exhibiting high lithium-ion-conducting properties at room temperature is generally limited to a liquid electrolyte. For example, an organic electrolytic solution is known as a material exhibiting high lithium-ion-conducting properties at room temperature.
A lithium-ion-conducting Li3N-based ceramic is also known which exhibits a high conductivity of 10−3 Scm−1 or more at room temperature.
However, a known organic electrolytic solution is inflammable since it contains an organic solvent. Therefore, leakage or ignition may occur when using an ion-conductive material containing an organic solvent as an electrolyte for a cell.
Moreover, since conduction of not only lithium ions but also counter anions occurs in a liquid electrolyte, the lithium ion transport number is not “1”.
Since a known lithium-ion-conducting Li3N-based ceramic has a low decomposition voltage, it is difficult to form an all-solid battery which is operated at 3 V or more using such a ceramic.
To deal with this problem, a sulfide-based crystallized glass has been disclosed which contains 50 to 92.5 mol % of Li2S and 7.5 to 50 mol % of P2S5, has a crystallization rate of 30 to 99%, and contains a glassy phase containing Li2S and P2S5 as the major components and a crystalline phase containing at least one compound selected from the group consisting of Li7PS6, Li4P2S6, and Li3PS4 (see JP-A-2002-109955, for example).
This sulfide-based crystallized glass exhibits high lithium-ion-conducting properties at room temperature.
However, this crystallized glass is heated at 500° C. or more during production and therefore requires special equipment for production on an industrial scale. Moreover, since a high ionic conductivity is obtained when the amount of Li2S is 80 mol % and the amount of P2S5 is 20 mol %, a large amount of expensive Li is used as a source.
Therefore, since the production cost of the material is increased, the above crystallized glass is not necessarily satisfactory from an economical point of view.
In addition, a material exhibiting higher lithium-ion-conducting properties has been demanded in order to improve the efficiency of a lithium rechargeable battery using a sulfide-based crystallized glass.
The invention was achieved in view of the above-described problems. An object of the invention is to provide a sulfide-based crystallized glass which exhibits high lithium-ion-conducting properties at room temperature, allows production on an industrial scale due to a decrease in the heat treatment temperature and a decrease in the amount of Li used as a source, and is satisfactory from an economical point of view.